That he has problems. Up to you to ask him what his deal is.

He's in denial. Maybe he still has feelings for you but dont wanna get into it again so he's trying to tell himself that he hates you. Because, in my experience, hate is not the opposite of love... rather, it is Indifference.

He still likes you. Or maybe he's just turning the tide, by saying he hates you its a start of conversation. And that means you start the talk and he waits.

That he still has feelings for you. By saying he hates you, hes trying to get his mind off of you, trying to forget you,depending on how long you guys went out, it may be harder if you guys are friends.